This invention relates to an optical scanning device for scanning an optical record carrier, such as an optical disk, comprising an information layer, the device comprising a radiation source for generating a radiation beam and an objective lens, located in an optical path between the radiation source and the information layer, for converging the radiation beam to a spot on the information layer.
There is a need for the production of optical record carriers of high capacity. Therefore, optical scanning devices using a relatively short wavelength radiation beam, for example a radiation beam of 400 nm, and a high numerical aperture (NA) objective lens system, with say NA greater than 0.7 and for example NA=0.85, are desirable.
In a known optical scanning device providing a relatively high NA beam at the location of the optical disk being scanned, a compound objective lens is used to provide multiple-stage condensing of a generally collimated beam originating from the radiation source. In such a system, it is known to mechanically adjust the spacing of the two, or more, lens elements of the compound objective lens, in order to compensate for spherical aberrations generated by different optical path lengths (referred to herein as information layer depths) through which the beam travels in an optical disk to reach an information layer in the disk. Another method of compensation is by mechanically adjusting the position of the collimator lens with respect to the radiation source, so that the radiation beam impinges on the objective lens as a convergent, or divergent, instead of collimated, beam. Each of these methods alters the amount of spherical aberration generated in the optical system of the scanning device, to correctly cancel out that generated in the optical disk being scanned. A separate mechanical actuator is used to provide focus control to maintain focus of the beam to a spot on the information layer being scanned.
A known optical scanning device is described in U.S. Pat. No. 5,889,789. The device includes means for compensating spherical aberrations in the form of a controller, switchable between a setting for an optical disk having an information layer depth of 1.2 mm and a setting for an optical disk having an information layer depth of 0.6 mm. In one setting, a planar plate is inserted, by a mechanical actuator, into the optical path of the radiation beam. In the other setting, the planar plate is removed, by the mechanical actuator, from the optical path of the radiation beam.
However, using mechanical actuators to provide spherical aberration compensation, particularly when a separate mechanical actuator is used to provide focus control, is relatively complex and therefore increases the cost of manufacture of the scanning device.
It is possible to do without a spherical aberration compensation subsystem; however, it is necessary to maintain strict tolerances in the manufacture of optical disks used with the optical scanning device. Such tolerances are particularly strict when considering a device providing a relatively high NA scanning beam at the location of the optical disk. For example, an NA of 0.85 allows for a manufacturing tolerance in the information layer depth, which may be for example 0.1 mm, of approximately xc2x13 xcexcm. By controlling such manufacturing tolerances, it is possible to use a rigid compound objective lens to provide a high NA scanning beam and no spherical aberration compensation subsystem. However, controlling manufacturing tolerances to such a degree has the consequence of increasing the cost of manufacture of the optical disks, and furthermore does not allow for spherical aberration compensation during the scanning of multi-layer optical disks.
A yet further known optical scanning device is described in JP-A-9306013, in which a radiation beam from a semiconductor laser is converted into a circular, parallel beam and passed through a twisted nematic liquid crystal cell which selectively rotates the polarization of incident light by 90xc2x0. The beam is then passed through a phase adjusting member, to reduce spherical aberrations for recording mediums having different substrate thicknesses, towards an objective lens which focuses the beam onto the recording medium. The phase adjusting member adjusts the phase differently in a central part than in a peripheral part, and must therefore be a relatively complex optical element.
In accordance with one aspect of the invention there is provided an optical scanning device for scanning an optical record carrier comprising an information layer, the device comprising a radiation source for generating a radiation beam and a compound objective lens, located in an optical path between the radiation source and the information layer, for converging the radiation beam to a spot on the information layer, the objective lens including at least a first lens element arranged to converge the beam to a certain convergence and a second lens element arranged to converge the beam to a greater convergence, wherein the device comprises a spherical aberration compensation optical subsystem including an electro-optical element for altering an optical path length in a spherical aberration generating region, which region is located in the optical path between the first lens element and the location of the record carrier in the device.
Spherical aberration compensation may be provided for information layers at various depths within an optical disk, or between optical disks, even in a relatively high numerical aperture device, without the need for a mechanical system to provide such spherical aberration compensation.